1. Field of the Invention
The present invention relates to a swash plate type compressor adapted for being incorporated into a climate control system for automobiles, and more particularly, relates to a swash plate type compressor with shoes arranged between a swash plate and reciprocating pistons in such a manner as to be sufficiently lubricated by a lubricating oil suspended in a refrigerant gas during the compression of the refrigerant gas.
2. Description of the Related Art
Swash plate type compressors have been conventionally used in a climate control system for automobiles, in order to compress refrigerant gas which is a thermal exchange medium in the climate control system.
In the swash plate type compressor, for example, a pair of axially combined cylinder blocks are provided with a plurality of axial cylinder bores arranged to be parallel to one another while allowing double-headed pistons fitted therein to reciprocate for compressing the refrigerant gas delivered toward the climate control system. The compressor is further provided with a drive shaft axially extending through the combined cylinder block and arranged in parallel with the cylinder bores. The drive shaft is rotatably supported and has a middle portion to which a swash plate is secured so as to rotate together with the drive shaft.
The swash plate has opposite circular surfaces arranged so as to be inclined with respect to the axis of rotation of the drive shaft, and the marginal portion of the surfaces of the swash plate is engaged with the double-headed pistons via a rotation-to-reciprocation conversion means including shoes in the form of semispherical elements. Namely, the shoe of the rotation-to-reciprocation conversion means has a semispherical surface portion slidably engaged with a roundly recessed socket formed in a central portion of each piston, and an opposite flat surface portion slidably engaged with the marginal portion of the swash plate.
The swash plate rotating with the drive shaft is housed in a swash plate chamber centrally formed in the combined cylinder blocks, and the axially opposite ends of the combined cylinder blocks are closed by front and rear housings. Both housings are sealably attached to the ends of the combined cylinder blocks, via valve plates, and define therein a suction chamber for refrigerant gas before compression, and a discharge chamber for the compressed refrigerant gas.
With the described swash plate type compressor, when the drive shaft and the swash plate are rotated together about the axis of rotation of the shaft by an externally applied drive force, the opposite surfaces of the swash plate nutate so as to provide the respective double-headed pistons with an axial force via the shoes of the rotation-to-reciprocation conversion means, and accordingly, the double-headed pistons are reciprocated in the respective cylinder bores of the combined cylinder blocks.
During the operation of the compressor, all moving elements of the compressor such as the swash plate, the shoes, the reciprocating pistons, and ball and thrust bearings accommodated in the compressor are lubricated by lubricating oil contained in the refrigerant gas which is circulated through the climate control system, and is eventually introduced from an evaporator into the swash plate chamber of the compressor via a suction conduit. When considering the lubrication of the shoes, the flat surface portions of the respective shoes which are constantly in sliding contact with the marginal portions of the opposite surfaces of the swash plate must be sufficiently lubricated by the lubricating oil. Nevertheless, the lubricating oil suspended in the refrigerant gas introduced into the swash plate chamber is centrifugally dispersed away from the swash plate without wetting the contact portions of the flat surface portions of the shoes and the marginal portions of the swash plate. Accordingly, the flat surface portions of the shoes in sliding contact with the marginal portion of the swash plate is apt to become dry due to insufficient supply of the lubricating oil.
In order to solve this problem, for example, Japanese Unexamined Patent publication (Kokai) No. 57-49081 disclosed one proposal for providing a means for effectively lubricating respective flat portions of the shoes sliding on the marginal portions of the swash plate of a swash plate type refrigerant compressor. In accordance with the proposed means for effectively lubricating the shoes, the flat surface portions of the respective shoes opposite to the spherical portions are provided with a smooth bulged surface having an extremely large radius of curvature, namely, the bulged surface of each shoe is formed so that the maximum height thereof is equal to or less than 15 micrometers, preferably, approximately 2 through 5 micrometers. The bulged surface of the shoe in sliding contact with the swash plate contributes to formation of a thin wedge-like gap between the contacting portion of both elements, i.e., the shoes and the swash plate. Thus, when the lubricating oil contained in the refrigerant gas is attached to the swash plate,it is easily caught by the wedge-like gap during rotation of the swash plate so as to form a wedge-like oil film capable of constantly lubricating the contacting portions of the shoes and the swash plate. Consequently, seizure of the shoes can be prevented.
In the case of the swash plate type compressor discussed in Japanese unexamined Patent Publication (Kokai) No. 57-49081, the swash plate type compressor employs shoes having a diameter of 13.5 millimeters and provided with bulged sliding contact surface portions of which the radius of curvature was determined to be between 4,500 and 11,400 millimeters. Therefore, the maximum height of the bulged surface portion of the shoe is approximately 2 through 5 micrometers.
In spite of the above-mentioned provision of effective lubricating means for the shoes of a swash plate type compressor, the sliding contact portions between the shoes and the swash plate cannot be sufficiently lubricated when the compressor is running at a low speed, and accordingly, when the supply of the lubricating oil is absolutely reduced.